The measurement of damage initiation,particle adhesion and cohesive strength from traction displacement curves of a nano-toughened epoxy

In this work the deformation behaviour of a nano-toughened epoxy adhesive is measured at different levels of stress triaxiality. The test method consists of a notched axisymmetric adhesive layer loaded in tension. The recorded traction displacement curves were analysed numerically and it was found that the measured peak stress corresponds to the intrinsic cohesive strength, σ_max of the material. This method allows experimental measurement of σ_max for use in cohesive zone models of fracture. Additional features of the traction displacement curves include a kink that corresponds to particle debonding at a critical hydrostatic stress. By application of the Mori–Tanaka model, the relationship between the experimental measurements and particle/matrix adhesion is described.     

Load and temperature dependence of frictional force and frictional electrification in friction between polymer films obtained by simultaneous measurements

Simultaneous measurements of frictional force (F) and frictional electrification charge density (σ) were carried out for PS/Ny, PC/PET, and PS/PC polymer film sample systems in the temperature range of 10° to 100°C under various loads (W). Load dependence of F and σ were expressed by F = αWⁿ and σ = βW^m. Values of n and m changed with the number of rubbings and temperature. Rate of decrease in n became larger at about 90°C for all sample system. For PS/Ny systems, the values of m and n changed in a similar manner. This behavior was explained in terms of variations in true contact area. Values of F and σ were at maxima and/or began to increase at the temperatures corresponding to the glass transition temperature of the α-peak temperature in viscoelastic polymers. The maxima of F and σ may appear when the frequency of thermal motion of molecules taking part in the adhesion and separation process coincides with the frequency of mechanical motion of adhesion and separation (friction) between two surfaces, namely, when the true contact area becomes maximum.     

Effect of nano CuO addition on the tribo-mechanical behavior of alumina ceramics in non-conformal contact

Sintering of alumina from 1500°C to 1650°C and tribo-mechanical properties at room temperature had been investigated using nano CuO as a sintering aid. Bulk density gradually increases with sintering temperature from 1500°C to 1600°C and is optimized at 1600°C, beyond this, bulk density does not significantly increase at 1650°C. The addition of 2 wt% CuO showed the best result on densification. Densification of about 97.74% was attained at 1600°C with the incorporation of 2 wt% CuO. Nano CuO at grain boundaries forms CuAl₂O₄ liquid which modifies the morphology of the grain and improves mechanical properties. The formation of self-lubricating tribo-film on the wear track results in a low coefficient of friction <0.2 and reduces specific wear rate. 4 wt% CuO addition increases contact tensile stress (σ_max) by 51.2% and high Hertzian contact pressure (P_max≈1.51 GPa) causes plastic deformation of wear track. The re-solidified strengthening bond phase on the wear track simultaneously increases in friction coefficient and wear resistance with CuO addition. The optimizing effect of CuO addition shows that 2 wt% significantly decreases wear rate, and increases hardness and fracture toughness.     

Interfacial interactions and properties of cellular structured polyurethane nanocomposite based on carbonaceous nano-fillers

In this article, we have studied the effect of carbonaceous nanofillers viz. fullerenol (0D), carboxylated multi-wall carbon nanotube (MWCNT, 1D), hydroxylated graphene (2D) and combination of carboxylated CNT and hydroxylated graphene as 3D in thermoplastic polyurethane on the tensile properties of the fabricated cellular structures. The concentration of nano-fillers was varied as 0.1, 1, and 5 wt%. Tensile properties of the nanocomposite cellular structures were measured as per ASTM D882 at 20°C (below glass transition temperature, T_g) and 40°C (above T_g). The results have shown that the tensile strength was found to increase by 200%–300% and the tensile modulus was found to increase by 150%–300% for 2D and 3D nano-fillers while significantly poor results were observed for 0D. However, the test data tensile strength and modulus showed marginal increase at 20°C and marginally low at 40°C for 1D filler. The interfacial adhesion was calculated by using experimental tensile data and the predictive models. The interfacial adhesion parameter (B_σ) calculated using Pukanszky equation was found significantly higher value for 2D (B_σ20 = 195.8) and 3D (B_σ20 = 192.0) fillers while poor adhesion was observed for 0D (B_σ20 = −81.6) fillers. The developed cellular structured materials were also evaluated by attenuated total reflection Fourier transform IR spectra, differential scanning calorimetry, X-ray diffraction, scanning electron microscope, and transmission electron microscope.     

Effect of cooling rate and crack propagation direction on the mode 1 interlaminar fracture toughness of biaxial noncrimp warp‐knitted fabric composites made of glass/PP commingled yarn

The mode 1 interlaminar fracture toughness of biaxial (±45°) noncrimp warp‐knitted fabric composites made of glass/PP commingled yarn was investigated. The crack propagation along the warp and weft directions, respectively, was considered for the composites cooled at two different rates during laminate molding. The interlaminar fracture toughness was characterized by determining the critical strain energy release rate (G_IC) of initiation and propagation measured from the double cantilever beam tests. In the case of a slow cooling rate (1°C/min), most specimens possess pure interlaminar crack propagation and direction‐independence characteristics. Nevertheless, the high‐cooled (10°C/min) specimens fractured in both directions suffer extensive intraply damage (crack branching, debonding, and bridging of 45°‐oriented interfacial yarns) and knit thread breakage, leading to G_IC of propagation two times higher than that of the slow‐cooled specimens, and the clear difference in the G_IC values of initiation between the two directions may be due to the contribution of the knit thread breakage to the fracture energy. POLYM. COMPOS., 2008 © 2007 Society of Plastics Engineers     

Effect of Temperature on Probe Tack Adhesion: Extension of the Dahlquist Criterion of Tack

At a molecular level adhesive joint strength of pressure-sensitive adhesives (PSAs) is governed by the ratio between two generally conflicting factors: high energy of cohesive molecular interactions and large free volume. Increase in temperature leads to domination of the free volume contribution over the cohesive strength, affecting mechanisms of the debonding process, examined with a probe tack test. Linear viscoelastic properties and probe tack adhesion of five types of PSAs have been studied: polyisobutylene (PIB); acrylic, styrene-isoprene-styrene (SIS) triblock copolymer; hydrogen-bonded complex of high molecular weight poly(N-vinyl pyrrolidone), PVP; with oligomeric poly(ethylene glycol), PEG; and plasticized polybase—polyacid polyelectrolyte complex (PEC). The transition from solid-like mechanism of debonding to ductile type of adhesive bond failure with fibrillation of adhesive layer has been established to occur for all examined PSAs under temperature increase within the range from ?20 to 80°C. The Dahlquist criterion of tack, which defines the value of the storage modulus, G′, below 0.1 MPa, featured for all the PSAs demonstrating maximum work of debonding, has been found to have a universal character and holds at corresponding temperatures for all the PSAs examined, including both typical and innovative adhesives. In addition to this adhesion predictor we have also established that for all the PSAs the transition from a solid–like debonding mechanism to a ductile type of debonding is observed in the range of G′ = 0.09–0.34 MPa. The value of the dissipation factor, tan δ, is also included in the analysis of correlation between linear viscoelasticity and probe tack behavior.     

Adhesion promotion between a homopolymer probe and a glass substrate coated with a block copolymer monolayer

The adhesion between a homopolymer matrix and a diblock copolymer is shown to depend on the length of the non-adsorbing block that penetrates the matrix chains. The tack or short-time adhesion was measured using a probe-tack set-up consisting of a thick (∼100 μm) polystyrene, PS, adhesive layer with degree of polymerization (DP), 1923, brought into contact with a monolayer of poly(deuterated styrene-block-methyl methacrylate), dPS-b-PMMA, deposited on a glass substrate. Experiments performed at 130 °C, above the glass transition temperatures, show that the maximum debonding stress increases from 1.4 MPa for the glass to 2.3 MPa for the copolymer. Also, the adhesion energy increases dramatically as the non-adsorbing block length, N_dPS, increases from below to above the entanglement DP of PS. These observations suggest a difference in the debonding mechanism between the nude glass, which undergoes fragile rupture, and the glass covered by dPS-b-PMMA, which exhibits increasing cavity formation with increasing N_dPS. After normalizing by the chain areal density, the adhesion dissipation is observed to increase by a factor of 4 as N_dPS increases from 100 to ca. 1000. These results suggest that entanglements between matrix chains and the non-adsorbing block impart good stress transfer and interfacial strength across the interface.     

Electric field tunable thermal stability of energy storage properties of PLZST antiferroelectric ceramics

The electrical hysteresis behaviors and energy storage performance of Pb_0.97La_0.02(Zr_0.58Sn_0.335Ti_0.085)O₃ antiferroelectric (AFE) ceramics were studied under the combined effects of electric field and temperature. It was observed that the temperature dependence of recoverable energy density (W_re) of AFE ceramics depends critically on the applied electric field. While W_re at lower electric fields (<8 kV/mm) shows increasing tendency with increasing temperature from 20°C to 100°C, W_re at higher electric fields (>8 kV/mm) demonstrates decreasing dependence. There exists an appropriate electric field (8 kV/mm) under which the AFE ceramics exhibit nearly temperature‐independent W_re (the variation is less than 0.5% per 10°C). The underlying physical principles were also discussed in this study. These results indicate that the temperature dependence of W_re of AFE materials can be tuned through selecting appropriate electric fields and provide an avenue to obtain thermal stable energy storage capacitors, which should be of great interest to modern energy storage community.     

Properties of molded soy protein isolate plastics

Thermal property of soy protein isolates (SPI) was studied with differential scanning calorimetry and thermogravimetric analysis. The weight loss of pure SPI is about 300°C. The glass transition temperature (T_g) is above 200°C. The best molding temperature of glycerin plasticized SPI plastics were then given. It is between 125 and 140°C. Subsequently the special property of molded SPI plastics was investigated. Results show that the atmosphere humidity affects the mechanical property and thermal property of SPI plastics. With the increasing humidity, the tensile strength decreases. While the elongation at breakage and peak area of the differential scanning calorimetry curve increases. At high temperature even at 140°C the molding temperature SPI plastics still have tensile strength though it decreases with the increasing test temperature while elongation at breakage increases. Dynamic mechanic thermal analysis test show that the storage modulus decreases with the rising temperature. The mechanical loss peak appears at lower temperature with the increasing amount of glycerin content. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Velocity measurements inside the concentration boundary layer during copper-magneto-electrolysis using a novel laser Doppler profile sensor

The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. With this new sensor, time-resolved velocity measurements within 400 μm of a vertically aligned cathode in an aqueous 0.05 M CuSO₄-solution are presented. In this way, the complex interaction of Lorentz force and opposing buoyancy-driven convection was studied by measuring the resulting velocity profile inside the concentration boundary layer with a spatial resolution of 15 μm. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode. The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. Its characteristic quantities, given by the horizontal distances, δ_max and δv=0, where the velocity reaches the maximum and where it is equal to zero, remain nearly unchanged, while the maximum velocity, vmax, in spite of the counteracting Lorentz force, increases faster as compared to pure natural convection, depending on the current density.     

Mixed modifier effect in lithium‐calcium borosilicate glasses

The mixed modifier effect (MME) in the lithium‐calcium borosilicate glasses, which have a composition of 0.4[(1?x)Li₂O–xCaO]–0.6[(1?y)B₂O₃–ySiO₂] with x in the range of 0~1 and y in the range of 0.33~0.83, is investigated. The MME manifests itself as a positive deviation from linearity in the activation energy of electrical conductivity (Ea^σ) and as a negative deviation from linearity in the fraction of four‐coordinated boron (N₄), glass transition temperature (T_g), dilatometric softening temperature (T_d), Vickers microhardness (H_v), dielectric constant (ε), and dielectric loss (tanδ). Moreover, the deviation, which exhibits a maximum at [CaO]/([CaO]+[Li₂O])=0.5, is enhanced with increasing [SiO₂]/[B₂O₃] ratio in the glass network. The observed MME in T_g, T_d, and H_v are attributed to the bond weakening in the network; however, the MME in ε, tanδ, and Ea^σ are caused by the obstruction of modifier transport in the glass network.     

Experimental characterization of interfacial adhesion of an optical fiber embedded in a composite material

The efficiency of an optical sensor embedded in a composite structure strongly depends on the interfacial adhesion between the optical fiber coating and the surrounding solid material. The present paper reports on the study of the interfacial adhesion of an optical fiber embedded in a composite material. A simple system composed of optical fibers embedded in an epoxy vinylester resin was first studied to evaluate the influence of embedded length, curing temperature and curing time. Pull-out tests on optical fibers bonded in epoxy vinylester/glass fiber composite material were carried out to measure the effect of glass concentration on the fiber bonding. The pull-out results showed no effect of both embedded length and curing temperature. However, an increase of the interfacial debonding stress is reported with increased curing time. For the optical fiber/composite system, a linear evolution of interfacial debonding stress with increasing glass fiber concentration is reported.     

Isocyanate trimerization kinetics and heat transfer in structural reaction injection molding

Guidelines are developed for molding large composite parts via structural reaction injection molding using glass preforms and polyisocyanurate resins. These are based on numerical simulations of the simultaneous heat transfer and reaction kinetics of a commercial system during and after mold filling. Premised requirements are that resin does not gel before the mold is filled, yet, reactions are sufficiently vigorous to approach completion. An existing mechanistic kinetic model is used and material parameters found from a chemical kinetics study employing an insulated cup. It is found desirable to use a high mold temperature and a low preform temperature in molding. Nondimensionalization of the governing equations reveals the existence of a Nusselt number (N_u), which describes the relative importance of heat transfer between resin and glass relative to thermal diffusion to the mold wall. With a Nusselt number of about 50 or higher it is possible to use the cooling capacity of the preform to extend gel time. The magnitude of N_u is influenced by part thickness, glass fraction, strand diameter, and flow velocity. Thus, the effect of the preform on extending resin gel time is within control of the molder.     

Effect of hygrothermal conditioning on the energy release rate and failure mechanism of metal only adherend-glass fibre prepreg co-cured single lap joints

Abstract

Fibre-reinforced composite materials are extensively used in repair and rehabilitation of oil and gas metal infrastructures which are largely exposed to water and hydrocarbon. An important aspect to this is applying adequate surface preparation to the metal to ensure a durable bond between the composite and metal substrate. In this paper, mild steel surface was prepared using grit blasting and single lap joint (SLJ) test specimens were manufactured and tested to investigate the adhesion in terms of total energy release rate (G_T) of the interface between mild steel adherend and glass fibre prepreg. An out-of-water usable epoxy resin primer was incorporated to join mild steel adherend with glass fibre prepreg by curing at a temperature of 55 °C for 48 h. Upon durability testing of the SLJ specimens using hygrothermal conditioning at a temperature of 55 °C for 1000 h, the experimental G_T values were seen to reduce significantly. Comparatively lower amount of cohesive failure and increased amount of swelling or delamination of the adhesive was observed for conditioned SLJ specimens when compared to controlled SLJ specimens. Furthermore, the experimental G_T values were found to correlate well with an analytical adhesive interface model.     

Effects of high energy ball milling on mechanical and interfacial properties of PBT/nano-Sb2O3 composites

In this work, the poly(butylene terephthalate) (PBT) nanocomposites containing modified nano-Sb₂O₃ particles were dispersed by two different dispersing techniques, including high speed rotating to disperse (HSR) and high energy ball milling to disperse (HEBM). The dispersion, interfacial interaction and mechanical properties of nanocomposites were investigated. The results showed that the dispersion and compatibility of nanocomposites dispersed by HEBM were better than that of HSR. From the analysis of interfacial interactions between nano-Sb₂O₃ particles and PBT matrix, the interfacial adhesion (B) and tensile strength of interfacial (σ_i) were decreased with the increase of nano-Sb₂O₃ particles content. The value of parameters B and σ_i of HEBM nanocomposites was higher than that of HSR, which indicated that the nanocomposites dispersed by HEBM had stronger interfacial interaction than that of HSR. As a result, the nanocomposites dispersed by HEBM had better mechanical properties than that of HSR.     

Effect of zinc stearate on properties of melt processable ionomer based on sodium salt of sulphonated maleated EPDM rubber

Abstract

Sulphonation of maleated copoly (ethylen/propylen/diene), followed by its neutralisation by sodium hydroxide produces an ionomer containing both carboxylate and sulphonate anions on the backbone. Addition of zinc stearate lowers the melt viscosity of the ionomer, which is higher than the corresponding non-ionomer. Dynamic mechanical thermal analysis shows that zinc stearate acts as a low reinforcing filler under ambient conditions and as a plasticiser above 100°C (i.e. above the melting point of zinc stearate). For example, incorporation of zinc stearate causes an increase in storage modulus E′ at 25°C, but a sharp decrease in E′ at 110°C. Furthermore, the plot of tan δ v. temperature reveals that tan δ at the low glass–rubber transition temperature T _g decreases, while tan δ at the high temperature ionic relaxation temperature T _i increases in the presence of zinc stearate. Incorporation of carbon black lowers tan δat T _g and increases tan δ at T _i, thus strengthening the biphasic structure of the ionomer. The ionomer shows higher tensile strength and modulus than the corresponding non-ionomer. Addition of zinc stearate increases the tensile strength and elongation at break, with marginal decrease in modulus. Carbon black increases the stress–strain properties of the zinc stearate filled ionomer. Reprocessability studies of the ionomer filled with zinc stearate and carbon black show that the material can be recycled without a decrease in properties.     

ELIMINATING FLOW INDUCED BIREFRINGENCE AND MINIMIZING THERMALLY INDUCED RESIDUAL STRESSES IN INJECTION MOLDED PARTS*

Eliminating flow-induced birefringence and stresses and reducing thermally induced stresses in the injection molded parts have been studied using rapid thermal response (RTR) molding technique. In the RTR molding, mold surface temperature can be rapidly raised above T _g in the filling stage, while the normal injection molding cycle time is still maintained. Therefore, the melt can fill the cavity at temperatures above T _g, which enables the flow-induced stresses to relax completely in a short time after filling and before vitrification. Residual stresses and birefringence in a RTR molded strip specimen are compared with the conventional molded parts by applying layer removal method and retardation measurement. For the material (Monsanto® Lustrex Polystyrene) and process conditions chosen, the birefringence level decreased as the RTR temperature approached and exceeded the glass transition temperature until it almost disappeared at a RTR temperature of 180°C. Reduction of magnitude and shift of peak location were observed in the gapwise stress profile for RTR molded specimen.

     

The application of an extrusion slit die in the rheological measurements of polyethylene composites with calcium carbonate using an in‐line rheometer

This article has reported the results of rheological testing of low‐density polyethylene (LDPE) and its calcium carbonate composites containing 7, 14, 21, and 28 wt% filler, respectively. The polymer composites were produced in a twin‐screw extrusion process. The assessment of the rheological properties of the polymeric materials was made under extrusion process conditions, using an in‐line rheometer with an extrusion slit die (W = 20, H = 2, L = 150 mm), at temperatures of 170°C, 180°C, and 190°C, respectively. The rheological parameters were determined based on the Ostwald‐de‐Waele power law model. The employed testing stand enabled the assessment of the effect of filler addition and slit die temperature on the variations in viscosity, power law index (n), consistency index (K), maximum flow velocity (V_max), and maximum flow profiles (V_z), under the conditions of technological processing (extrusion) of plastics. POLYM. ENG. SCI., 59:E16–E24, 2019. © 2018 Society of Plastics Engineers     

Experimental Investigation on the Damping Behaviors of Epoxies with Different Epoxy Value

In this article, five different epoxies including a new kind of flexible epoxy having low glass transition temperatures (11 ～ 28°C) were prepared using polyamine as curing agent. Damping mechanical tests show that compared with other common available epoxies, the flexible epoxy has high loss factor over broad frequency and temperature range. Activation energy corresponding to glass transition process of different epoxies has been calculated from the temperature corresponding to tan δ_max values, obtained at different measurement frequencies. The maximum value of loss factor is 0.71 and the Tg varies from 11 to 28°C, indicating the flexible epoxy can be used as damping polymer materials in common temperature or frequency range.     

First Principles Study of the Aluminum–Cubic Boron Nitride Interface

A plane wave density functional methodology, with the local density approximation for the elemental constituents, was used to investigate the structure, bonding, and adhesion of atomic-scale interfaces between aluminum and cubic-boron nitride (c-BN). Two fully periodic interfaces, Al(110)–c-BN(110) and Al(001)–c-BN(110), were constructed for this purpose. Interfacial bonding, examined with contours of the charge density difference and electron localization function, was found to be stronger between Al–N pairs than Al–B pairs. The computed work of separation (?W_s?) values were 2.25 J/m² for Al(110)–c-BN(110) and 2.65 J/m² for Al(001)–c-BN(110). The higher adhesion in the latter interface is attributed to a higher planar density of interfacial Al atoms. The computed W_s values were compared with values from first principles calculations on other aluminum–ceramic interfaces. The possibility of adhesive transfer during tensile debonding was qualitatively investigated.